Most television monitors have curved monitor display areas. After the completion of manufacture of the monitor, when a picture is first projected on the screen, its curvature distorts the picture. The technician needs to adjust the beam deflection circuitry to provide a flat, linear image with no geometric distortions.
The calibration is usually performed by generating a pattern, typically a crosshatch pattern, on the CRT screen. This pattern is then compared to a fixed reference pattern. The fixed reference pattern can be placed on a thin film which is then attached to the surface of the monitor. Another method of providing a fixed reference pattern has been to optically project it on the front surface of the monitor. The operator then adjusts the beam deflection circuitry until all crosshatch marks from the monitor display line up with all co-ordinates of the fixed reference pattern.
The methods for providing a fixed pattern have not proved entirely satisfactory. When using a film on the monitor surface, the alignment of the film is difficult to adjust. If the film is placed on a thick, transparent plate to aid in alignment, there are parallax problems when viewing the test pattern on the screen. Optically projecting a pattern on the monitor is also clumsy to use.